The ability of tumor cells to develop simultaneous resistance to multiple cytotoxic drugs constitutes a major problem in cancer chemotherapy. This multidrug resistant phenotype is due to decreased intracellular drug accumulation, apparently as a result of alterations in plasma membrane. It was recently demonstrated by us that multidrug resistance in cultured Chinese hamster cells correlates with amplification of specific DNA sequences. An amplified gene associated with multidrug resistance in these cells was isolated. Recently DNA sequences homologous to this gene were found to be amplified in leukemic cells of a chemotherapy-resistant patient. In order to investigate the genetic mechanisms involved in the development of multidrug resistance by human tumors, the human homologue of the Chinese hamster gene will be isolated, and its amplification and expression in multidrug-resistant human tumor cells will be analyzed. In addition, multidrug-resistant human tumor cells will be assayed for amplification of other genes, using the in-gel DNA renaturation technique developed by the P.I. This technique detects amplified genes in the absence of any preliminary information about their sequence or location in the genome. Any additional amplified genes found by this procedure will be cloned and analyzed. Amplification and expression of the cloned genes in chemotherapy-resistant human tumor samples will be correlated with the tumor drug resistance profiles. Tumors that have acquired drug resistance in the course of chemotherapy and tumors initially unresponsive to chemotherapy will be compared with regard to the expression of cloned genes. Expression of these genes in individual cells within the tumors will be analyzed by in situ hybridization. Full-length cDNA and/or genomic clones of multidrug resistance-associated genes will be isolated, and the functions of these genes will be analyzed by gene transfer. cDNA sequences of the cloned genes will be determined. The products of these genes will be expressed in bacteria. At a later stage, these products or chemically synthesized peptides will be used to raise antibodies against multidrug-resistant human tumor cells, with the aim of investigating the possible diagnostic and therapeutic applications for such antibodies.